Fig. 9. Schematic lateral view of the left side of a chub mackerel in which the body is outlined in gray and the finlets and caudal keels are outlined in black. Idealized flow trajectories (gray arrows) are shown for forward gliding (A, body motion indicated by the black arrow), in which there is no lateral movement of the caudal peduncle or finlets, and for lateral movement of the body without forward movement (B, body motion indicated by the black arrow). Forward gliding without lateral movement (A) is expected to create convergent flow along the peduncle. Lateral movement of the peduncle without forward movement (B) is expected to create divergent flow on the leading surface of the peduncle and convergent flow on the trailing surface. The observed flow pattern on a steadily swimming mackerel is a combination of these two idealized patterns. Above the lateral midline of the fish on the leading peduncular surface (shown), the downward trajectory of flow above the horizontal expected under A sums with the upward trajectory expected under B to result in essentially horizontal flow. The downward trajectory of flow on the trailing peduncular surface expected under A sums with the downward trajectory expected under B to result in flow above the horizontal moving towards the midline on the trailing surface. Flow below the horizontal follows the opposite pattern.